In a gold deposit near Nassara, southern Burkina Faso, gold occurs closely associated with pyrite within a network of veins hosted by metavolcanic and metasedimentary rocks. Using SEM and LA-ICP-MS analyses, we identified three generations of pyrite with distinct roles in gold mineralization. Pyrite 1 (Py1) formed early during mineralization, replacing alteration minerals like ankerite in metabasalt. Pyrite 2 (Py2) developed around Py1 in pressure shadows caused by localized micro-shear zone reactivation during successive micro-seismic events. Pyrite 2 is enriched in As and Au, unlike Py1. Pyrite 3 (Py3), unrelated to mineralization, formed at a later stage. Gold occurs in pyrite as micro-inclusions (in Py1 and Py2), fracture-fillings (mainly in Py2), and within the pyrite structure as invisible gold, including nanoparticles (predominantly in Py2). Combining electron backscatter diffraction (EBSD) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis reveals that deformation-induced misorientation of pyrite facilitated the remobilization of invisible gold, which subsequently re-precipitated as colloidal particles along sub-grain boundaries and within fractures, mimicking visible inclusions. These findings demonstrate that gold perceived as inclusions (visible or invisible) often precipitates within micro/nano-fissures and sub-grain boundaries during remobilization. This highlights the critical importance of thorough ore characterization for accurately determining gold deportment. Such insights advance our understanding of mineralization processes and support the development of more efficient recovery strategies.

Abstract. Geochemical and isotopic data from the Paleoproterozoic greenstone belt of Mako (Eastern Senegal) provide evidence for the existence of two tholeiitic and one calc-alkaline series. Tholeiitic series are composed of ultramafic and mafic rocks. Tholeiites 1 are characterized by flat Light Rare Earth Elements (LREE) and Middle Rare Earth Elements (MREE) patterns and by initial isotopic compositions of 87Sr/86Sr (2.1 Ga) from 0.70083 to 0.70368 and εNd (2.1 Ga) from +1.55 to +5.90 (except sample IL49). Tholeiites 2 are characterized by a more enriched LREE and MREE pattern, and by initial isotopic compositions 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70263 and εNd (2.1 Ga) from +2.14 to +7.35. In addition tholeiites 2 show a slightly negative Nb anomaly, while the tholeiites 1 have no Nb anomaly. Calc-alkaline rocks are characterized by more enriched LREE and MREE patterns and more depleted Heavy Rare Earth Elements (HREE), indicating more pronounced general LREE/HREE fractionation; they have a more pronounced negative Nb and Ti anomalies and initial isotopic signatures of 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70618 and εNd (2.1 Ga) from +3.12 to +6.40. Comparison of geochemical and isotopic data of the three magmatic series with those of other Paleoproterozoic domains of the West African Craton and juvenile volcanic rocks leads us to propose an evolutive model for the Mako series comprising an initial emplacement of an oceanic plateau, in which the first tholeiitic series was generated, followed by a subduction process that triggers the contamination of the mantle plume source and the generation of the second tholeiitic series at the earliest stages of the arc built; and ending with the maturation of an island arc and the calc-alkaline series emplacement, which has a chemical signature indicating that the source region was likely a metasomatized mantle probably mixed with some plume mantle source.

Sediment -hosted gold deposits represent a significant portion of the world's gold resources. They are characterized by the ubiquitous presence of organic carbon (C-org; or its metamorphosed product, graphite) and the systematic occurrence of invisible gold -bearing arsenian pyrite. Yet the role played by these features on ore formation and the distribution of gold remains a long-standing debate. Here, we attempt to clarify this question via an integrated structural, mineralogical, geochemical, and modeling study of the Shahuindo deposit in northern Peru, representative of an epithermal gold deposit contained in a sedimentary basin. The Shahuindo deposit is hosted within Lower Cretaceous fluvio-deltaic carbon -bearing sandstone, siltstone, and black shale of the Maranon fold -and -thrust belt, where intrusions of Miocene age are also exposed. The emplacement of the auriferous orebodies is constrained by structural (thrust faults, transverse faults) as well as lithological (intrusion contacts, permeable layers, anticlinal hinge in sandstone) features. The defined gold reserves (59 tons; t) are located in the supergene zone in the form of native gold grains. However, a primary mineralization, underneath the oxidized zone, occurs in the form of invisible gold in arsenian pyrite and arsenopyrite. Here, four subsequent pyrite generations were identified-namely, pyI, pyII, pyIII, and pyIV. PyI has mean Au concentrations of 0.3 ppm, contains arsenic that is not detectable, and is enriched in V, Co, Ni, Zn, Ag, and Pb compared to the other pyrite generations. This trace element distribution suggests a diagenetic origin in an anoxic to euxinic sedimentary basin for pyI. Pyrite II and pyIV have comparable mean Au (1.1 and 0.7 ppm, respectively) and As (2.4 and 2.9 wt %, respectively) concentrations and precipitated under conditions evolving from lower (pyrrhotite, chalcopyrite, sphalerite) to higher (enargite, digenite, chalcocite) sulfidation, respectively. The pyIII generation is the major gold event in the primary mineralization, with pyrite reaching 110 ppm Au (mean similar to 7 ppm) and 5.6 wt % As (mean similar to 1.8 wt %), while coeval arsenopyrite attains 460 ppm Au. Pyrite III is also enriched in other trace elements such as Se, Ge, Mo, In, Ga, and Bi compared to the other pyrite generations, which is indicative of a magmatic source. Bulk analyses of the surrounding unmineralized rocks show only parts per billion levels of Au and less than 25 ppm As. These data, combined with mass balance considerations, demonstrate that the sedimentary rocks could not be the sole source of gold, as they could only contribute a minor portion of arsenic and sulfur (and iron) to the deposit. Conversely, fluids exsolved from a pluton crystallizing at depth likely provided the great part of the gold endowment. Equilibrium thermodynamics simulations, using geochemical constraints established in this study, demonstrate that interaction between Au-As-S-Fe-bearing fluids and organic carbon -bearing rocks strongly enhanced the fluid ability to transport gold by maximizing its solubility as Au-I hydrosulfide complexes via a combined increase of pH and aqueous sulfide concentration. This finding challenges the traditional qualitative view of organic matter acting exclusively as a reducing agent for Au-I that should promote gold deposition in its native state (Au-0) rather than enhance its solubility in the fluid. Our results have significant implications for the exploration of carbonaceous sedimentary environments. Such settings may provide a very effective mechanism for focusing gold transport. Subsequent scavenging of AuI from solution in a chemically bound form is promoted by the precipitation of arsenian pyrite in permeable structural and lithologic traps, bound by more impermeable units, similar to what occurs in petroleum systems. Our integrated study underlines the important potential of sedimentary C-org-bearing rocks in the formation and distribution of gold and associated metal resources.

The Tabakoto gold deposit is part of the highly endowed west-Malian gold belt, which hosts several world-class deposits. Located in the Paleoproterozoic Ké dougou-Ké nié ba Inlier (KKI) of the West African Craton (WAC), the deposit is contained in Birimian metasedimentary rocks of the Kofi series that are intruded by magmatic dikes. The Tabakoto deposit is characterized by multiple overlapping stages of magmatic dike intrusions and hydrothermal alteration during distinct regional-scale deformation events. The metasedimentary rocks are characterized by a S0/1 foliation delineated by the alternation of metagreywacke and meta-argillite. This foliation is affected by upright folds with N-S trending axial planes marked by a S2 schistosity and cross-cut by conjugate steep-dipping dextral NE-SW and sinistral NW-SE trending faults. These structures record regional-scale E-W shortening, first associated with crustal thickening and then with N-S stretching, evolving from ductile to brittle deformation. The gold-bearing V2 quartz-pyrite veins developed in dikes and metasedimentary units, are parallel to the S2 schistosity of the DT2 deformation event. These veins contained reduced phases such as pyrrhotite, loellingite and scheelite as well as bismuth. The second gold-bearing V3a quartz-carbonate veins are located in the NE-SW and NW-SE trending faults of the DT3 deformation event. The latter veins are marked by sodic alteration followed by carbonate (i.e., dolomite-ankerite +/- calcite +/- siderite) and phyllic (i.e., chlorite-muscovite-sericite) alterations. In the quartz-pyrite veins, gold occurs as inclusions in pyrite, arsenopyrite, and pyrrhotite, whereas in quartz-carbonate shear veins, gold is present in microfractures cross-cutting pyrite and arsenopyrite and at the contact between grains of pyrite and arsenopyrite, and of arsenopyrite and pyrrhotite. The litho-structural and mineralogical features of the Tabakoto deposit portray a polyphase mineralization with (i) an intrusion-hosted orogenic gold system, where the metabasltic to metarhyolitic dikes acted as suitable emplacement for fluids flow and (ii) a hydrothermal fluid circulation under ductile to brittle conditions. Gé ologie du gisement d'or de Tabakoto, Boutonniè re de Ké dougou-Ké nié ba, Craton Ouest Africain, Mali. Le gisement d'or de Tabakoto fait partie de la ceinture aurifè re ouest-malienne trè s riche, qui abrite plusieurs gisements de classe mondiale. Situé au sein de la boutonniè re palé oproté rozoï que de Ké dougou-Ké nié ba (BKK) du Craton Ouest Africain (COA), le gisement est encaissé dans les roches mé tasé dimentaires birimiennes de la sé rie de Kofi qui est intrudé e par des dykes magmatiques. Le gisement de Tabakoto est caracté risé par de multiples é tapes superposé es d'intrusions de dykes magmatiques et d'alté ration hydrothermale au cours d'é vé nements de dé formation distincts à l'é chelle ré gionale. Les roches mé tasé dimentaires sont caracté risé es par une foliation S0/1 dé limité e par l'alternance de mé tagreywacke et de mé ta-argillite. Cette foliation est affecté e par des plis droits avec des plans axiaux orienté s N-S marqué s par une schistosité S2 et recoupé s par des failles cassantes conjugué es dextres à fort pendage orienté es NE-SW et senestres orienté es NW-SE. Ces structures té moignent d'un raccourcissement E-W à l'é chelle ré gionale, d'abord associé à un é paississement crustal, puis à un é tirement N-S, passant d'une dé formation ductile à une dé formation cassante. Les veines aurifè res de quartz-pyrite V2 sont dé veloppé es dans les dykes et les unité s mé tasé dimentaires et sont parallè les à la schistosité S2 de la phase de dé formation DT2. Ces veines contiennent des phases ré duites telles que la pyrrhotite, la loellingite et la scheelite ainsi que du bismuth. Les deuxiè mes veines aurifè res de quartz-carbonate sont situé es dans les failles orienté es NE-SW et NW-SE de la phase de dé formation DT3. Ces derniè res veines sont marqué es par une alté ration sodique suivie d'une alté ration carbonaté e (c.-à-d. dolomite-anké rite +/- calcite +/- sidé rite) et phyllitique (c.-à-d. chlorite-muscovite-sé ricite). Dans les veines de quartz-pyrite, l'or est pré sent sous forme d'inclusions dans la pyrite, l'arsé nopyrite et la pyrrhotite, tandis que dans les veines de quartz-carbonate, l'or est pré sent dans les microfractures recoupant la pyrite et l'arsé nopyrite et au contact entre la pyrite et l'arsé nopyrite ou entre l'arsé nopyrite et la pyrrhotite. Les caracté ristiques litho-structurales et miné ralogiques du gisement de Tabakoto montrent une miné ralisation polyphasé e avec (i) un systè me aurifè re orogé nique à intrusion, où les dykes mé tabasaltiques à mé tarhyolitiques ont servi d'emplacement approprié pour l'é coulement des fluides et (ii) une circulation de fluides hydrothermaux dans des conditions ductiles à cassantes.

Within southern Mali, the Syama belt constitutes a linear major structure-oriented N-S, which host several gold deposits (e.g., Syama and Tabakoroni) and prospect areas (e.g. Tellem). The Syama Belt is formed by magmatic rocks (basalts, lamprophyres, andesites, dacites and microgranites); sedimentary rocks (shales) and volcano-sedimentary rocks (pyroclastics). The magmatic rocks are divided into two main volcanic series: tholeiitic affinity rocks (basalts and lamprophyres) and calc-alkaline affinity (andesites) that are the most evolved. The field relationships between rocks of these two series suggest that the calc-alkaline series are younger the tholeiitic series. These tholeiitic series present the Mid-Ocean Ridge Basalt (MORB) affinity whereas the calc-alkaline series would be linked to an island arc-type. This coexistence is not an isolated case within the West African Craton (WAC). Otherwise, the Syama belt has all the characteristics of other belts, within which a number of gold deposits are developed, in the WAC.

Iron production has played a key role in the history of Africa for more than 3,000 years. The study of this human activity has demonstrated its exceptional significance, its historical antiquity and an astonishing variability of practice. Indeed, metallurgists developed different ways to smelt iron ores in Sub-Saharan Africa. They multiplied the technical choices to a degree unequalled on other continents.The region of Bassar (northern Togo) is known to be one of the oldest-ironmaking began there in the 5th century BCE-and most important iron production centers in West Africa -approximately 50,000 tons of iron were produced there between the 13th and 20th centuries. Recent research conducted within the international and interdisciplinary program SiderEnT (2014-2018) revealed the existence of several iron smelting technics there. In order to understand which factors could be at the origin of this, we have taken the most holistic approach possible. Thus, after establishing an exhaustive assessment of the geological, archaeological, archaeometric and ethnological data at our disposal, we evaluate the different possibilities that can explain the plurality of practices and then we propose different historical-cultural scenarios tracing the evolution of iron smelting technics in the Bassar region.

Iron production has played a key role in the history of Africa for more than 3,000 years. The study of this human activity has demonstrated its exceptional significance, its historical antiquity and an astonishing variability of practice. Indeed, metallurgists developed different ways to smelt iron ores in Sub-Saharan Africa. They multiplied the technical choices to a degree unequalled on other continents.The region of Bassar (northern Togo) is known to be one of the oldest ‒ ironmaking began there in the 5th century BCE ‒ and most important iron production centers in West Africa ‒ approximately 50,000 tons of iron were produced there between the 13th and 20th centuries. Recent research conducted within the international and interdisciplinary program SidérEnT (2014-2018) revealed the existence of several iron smelting technics there. In order to understand which factors could be at the origin of this, we have taken the most holistic approach possible. Thus, after establishing an exhaustive assessment of the geological, archaeological, archaeometric and ethnological data at our disposal, we evaluate the different possibilities that can explain the plurality of practices and then we propose different historical-cultural scenarios tracing the evolution of iron smelting technics in the Bassar region.

In the Echassieres district of the French Massif Central, occur several outstanding magmatic/hydrothermal systems enriched in strategic metals, such as the Beauvoir rare-metal granite. In this contribution we propose a systematic approach, based on mica trace chemistry, to decipher the different events leading to mineralization. Twelve groups of micas were defined by their specific petrographic features and/or location in the district. Their trace element composition, obtained by LA-ICP-MS, varies widely from one mica group to another, although homogeneous signatures within groups could be distinguished. Some of the trace elements are remarkably enriched, such as W in igneous lepidolite and Sn in greisen muscovite, both of which occur in the Beauvoir granite. A statistical approach based on a set of multivariate analyses highlights that the trace chemistry of micas is inherited from their source, whether hydrothermal or igneous, thus providing a signature for their origin. This approach also shows that differences in major element composition (i.e., different mica species) impact only slightly the trace-element signature. For instance, muscovite and zinnwaldite from one granite have a coherent signature, but they contrast with same mica species in another granite or hydrothermal veins. It is thus possible to genetically link two different mica species from remote locations, or inversely, to recognize different origins for a same mica species in the same sample (e.g., superposed alterations). A second, important implication is that trace-element signatures of micas provide a record of metal remobilization and transport. In the Echassieres district, greisen alteration of the Beauvoir granite caused dissolution of W-rich (ca. 290 ppm in average) lepidolite and cassiterite (SnO2). Newly-formed greisen muscovite incorporated most remobilized Sn (ca. 1000 ppm in average), while W precipitated in distal quartz veins as wolframite. As a consequence, Sn is concentrated in the granite, while W occurs outside of it. This is also underlined by the gradual Sn decrease and W increase recorded in micas from distal veins. Finally, wolframite-bearing veins do not contain cassiterite, validating mica trace-element chemistry as a powerful tool to decipher Sn-W ore forming hydrothermal processes.

The Yaou deposit, located in French Guiana within the Guiana Shield, is one of the most promising gold deposits of the regional Paleoproterozoic greenstone belt. It displays numerous quartz monzodiorite bodies aligned along a sinistral shear zone where a five-deformation phases model is established at the camp scale. The ductile D1/2YA phase is responsible for the main penetrative foliation while the D3YA phase is related to shearing. An intrusive event is identified as being pre to syn-D3YA. The following phase D4YA represents a brittle quartz-carbonate veining set hosted preferentially within intrusive bodies and along the shear zone. A local D5YA brecciation event crosscuts the D4YA veins. Among this deformation history, two auriferous events (D3YA and D4YA) control the overall grade of the Yaou gold deposit. More specifically, most of the Au grade is associated with the main economic D4YA veining event, where the gold is visible and linked to Py4 within an ankerite/hematite rich alteration halo. At the microscopic scale, results of in situ analyses using LA-ICP-MS on pyrite show that metasediment-hosted Py0 is a primary source of submicroscopic gold having a low contribution to the total endowment. Py3 shows some gold content due to possible remobilization of AuD0YA. Gold in Py4 is found as submicroscopic gold, as micro-inclusions and as infilling fractures in association with elements such as Te, Ag and Bi. Most contribution to the Au grade is from micro-inclusions and, to a lesser extent, from free and submicroscopic gold. The ore shoot locations are lithologically controlled for AuD0YA (metasedimentary unit-hosted), structurally controlled (shear zone-hosted) for AuD3YA and rheologically controlled for the AuD4YA (intrusion-hosted). The deposit is clearly polyphase both at the macroscopic and the microscopic scales, invisible gold is associated with As whereas visible gold is observed as inclusions in pyrite with high contents of Ag, Te and Bi. We define an early low-grade enrichment of AuD0YA to AuD3YA followed by a later high-grade event, AuD4YA supporting polyphase mineralization processes. This study confirms that orogenic gold deposits can be formed by remobilization and/or new gold inputs during multiple deformation, veining and hydrothermal events.

Pyrite (FeS 2 ), arsenopyrite (FeAsS) and löllingite (FeAs 2 ) are exceptional gold concentrators on Earth; yet the exact redox and structural state of this “ invisible ” gold and the forces driving its intake and release by these minerals remain highly contro-versial. Here we applied high resolution X-ray absorption spectroscopy to Au-bearing pyrite and iron sulfarsenides from hydrothermal deposits and their synthetic analogues. We show that Au preferentially enters octahedral Fe structural sites [Au(As,S) 6 ] enriched in As,by forming respectively[AuAs 1 – 3 S 5 – 3 ], [AuAs 3 S 3 ⋯ AuAs 6 ] and [AuAs 6 ] atomic units in arsenian pyrite ( > 0.1 – 1.0 wt. % As), arsenopyrite and löllingite, implying a formal oxidation state of Au II in the minerals. In contrast, in As-poorpyrite,Au is dominantly chemisorbedas [Au I S 2 ] moieties in muchlower concentrations. Combined with experimental data on Au mineral-fluid partitioning, our findings imply a universal control exerted by arsenic on gold incorporation in iron sulfides and sulfarsenides via coupled Au-As redox reactions. These reactions account for the observed variations in invisible gold contents in the minerals from different hydrothermal deposit types and enable quantitative prediction of iron sulfarsenide ability in controlling gold concentration and distribution in hydrothermal systems. and average bond lengths involving Au atom (in Å, (cid:69) angle in degree) of gold-bearing pyrite, arsenopyrite, and löllingite models. Angles other than the (cid:69) arsenopyrite angle are all equal to 90.0°. The reported interatomic distances are average values over the 1 st or 2 nd shell atomic cluster.

Using in-situ LA-ICP-MS, the trace-element composition of hydrothermal quartz from five Sb mineralized Variscan districts, from different parts of France, were compared. Each of these districts is characterized by the presence of stibnite, with quartz being a ubiquitous gangue mineral. The resulting dataset was processed using a set of multivariate statistical analyses (i.e. principal component analysis, hierarchical cluster analysis, and permutational multivariate analysis of variance). This approach highlights differences in ore-forming processes (e.g., fluid chemistry), recorded in the trace-element pattern of gangue minerals. Using this methodology, we could distinguish different mineralizing events with intricate paragenesis, such as those in the Sioule nappe, which appear to have recorded two different events of Sb precipitation. Results indicate that Sb incorporation in quartz is weakly correlated to other trace elements, but that its contents increase sharply in quartz from stibnite bearing veins. In this case, Sb shows values >10 ppm and up to 200 ppm, considerably higher than the values measured in quartz unrelated to Sb mineralization (mainly <= 1 ppm). Intermediate Sb contents in quartz would point to the potential for a fluid to precipitate Sb ore. It is concluded that incorporation of Sb in quartz can be confidentially correlated to its content in the mineralizing fluid. Based on our data from five Sb districts in the French Variscan belt, supplemented by published Sb trace-element data in quartz from a number of other Sb deposits around the world, this study highlights the strong potential of the Sb content of quartz as a proxy for tracing Sb mineralization.

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Despite its high endowments, the gold mining potential of Central African greenstone belts is seemingly underrated when compared to equivalent belts in neighbor West Africa. This is probably because, over the past half century, only minor exploration efforts were ever made in this region. In the southwest of the Central African Republic, near the locality of Moboma, gold-bearing quartz veins are hosted in greenschist facies Paleoproterozoic formations that are intruded by numerous dolerite dykes. These rocks occur in a strongly deformed terrane that marks the front of the Panafrican Oubanguides nappe, developed during an E-W regional shortening. Presence of multiple banding indicates repeated reactivation of the quartz veins and circulation of H2O-CO2-NaCl fluids, similar to those characterizing typical orogenic gold-bearing settings. Fluid inclusion petrography and microthermometry permitted to distinguish two different fluids: one, aqueous-carbonic, circulated at relatively high temperature (Th = 250-270 degrees C) and was responsible for the main stage of Au deposition; a second fluid of low-salinity trapped in microcracks and in a late quartz generation, interpreted as meteoric, precipitated silver-poor native gold. At a later stage, supergene alteration caused the formation of discrete gold nuggets in the upper levels of the mineralization. The competent nature of the dolerite dykes and quartzite intersected by these quartz veins contributed to focus rock fracturing, localizing fluid circulation and the mineralization. The alteration assemblage developed in the veins is equivalent to that found in the dolerite dykes, which was dated at 571 Ma, thus pointing to a Panafrican age for the mineralization at Moboma.

Les terres rares (TR) sont un groupe de metaux utilises dans de nombreuses nouvelles technologies. Leur production est limitee, il est donc important de trouver de nouvelles sources d'approvisionnement. Les TR sont divisees en TR legeres et lourdes, ces-dernieres etant plus rares et ayant plus d'applications. Les granites et pegmatites alcalins sont des roches riches en TR lourdes. Une partie de cet enrichissement est d'origine magmatique, mais le role des fluides hydrothermaux (i.e. eau, CO2, methane surtout) pose question. Ce manuscrit se concentre sur l'etude de mineraux communs (amphiboles, pyroxene, zircon) et des inclusions fluides de six zones du monde pour comprendre l'impact des fluides hydrothermaux par rapport a celui des processus magmatiques dans les granites et pegmatites alcalins. Les resultats montrent que la concentration et le fractionnement des TR sont lies a la circulation de fluides hydrothermaux riches en Na et K ou Ca et Na a des temperatures inferieures a 400 °C.

The Echassieres district of central France hosts diverse magmatic and magmatic- hydrothermal deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization crops out at three distinct wolframite occurrences, the two most important of which are related to two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature range (T-h approximate to 380 degrees C). No evidence for fluid pressure variations was observed, and temperature decrease is considered to be the main cause for wolframite deposition. The younger W mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several quartz (+/- topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, Th are spread from ca. 190 to 400 degrees C, and several populations exhibit heterogeneous liquid/vapor ratios while others consist of only vapor-rich fluid inclusions. Respectively, these populations are interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast with the other regional veins, flashing was particularly intense in the Mazet veins, which host the bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor that triggered W precipitation. This work highlights the role of two physical parameters, pressure and temperature, whose variations played a preponderant role on wolframite mineralization. It documents, in depth, an example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic fluids, and on greisen-related rare-metal deposits.

A rescue excavation at the site of Centre mobilisateur 10, located in the south-east of the Lannemezan agglomeration (Hautes-Pyrénées) and otherwise known as the Arsenal, was carried out in 2016 by the Éveha team, under the leadership of V. Ropiot. From a geographic perspective, this sector, between Bigorre and Comminges, occupies an interface position on a fluvio-glacially formed plateau, culminating at an altitude of 600 m at the foothill of the Pyrenees, and forms a vast expanse of land including heaths, bogs, meadows and some woodland.The excavation completely uncovered a circular tumulus with a pebble enclosure, which was the main aim of the operation. However, it also brought to light ten archaeological structures, including a singular megalithic monument, composed of a monolithic slab surrounded by a pebble border, covering a chamber buried in a vast oblong pit, with a frontal entrance.The other structures consist of six fireplaces with heated pebbles, two isolated post holes and a pit containing middle Bronze Age objects, which were plundered or heavily reworked but appear to be linked to worship rituals. All these structures are located in the central part of the excavated zone, around the tumulus, and towards the east for the hearths, including two developed in the crown of the tumulus. These remains are attributed to a period ranging from 3700 to 1400 BCE, centred around the Final Neolithic 2. They form an unprecedented monumental funerary complex in the High-Pyrenees.

The Kiaka orogenic gold deposit (Burkina Faso), located in the Paleoproterozoic domain of the West African Craton, is characterized by a two-stage gold mineralization hosted in volcano-sedimentary metamorphic rocks that was formed during the Eoeburnean (2.20–2.13 Ga) and Eburnean (2.13–2.05 Ga) orogenic cycles. These two stages include an early disseminated low-grade gold mineralization and a late vein-hosted high-grade gold mineralization. Paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline. The aim of this paper is twofold: (i) to determine the processes responsible for deposition of the economic disseminated gold mineralization and (ii) to identify the source of the mineralizing fluids. For this purpose, we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes. Hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different δ 11 B values falling within the two intervals of − 25.1 to − 22.0‰ and − 19.6 to − 15.1‰, respectively. Our results suggest that tourmaline formed from a distal, high-temperature (ca. 400 °C), reduced, and low-salinity hydrothermal fluid that interacted with the local host rocks. Based on the modeling of tourmaline–fluid boron isotope fractionation, we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca. 2.13 Ga. This metamorphic fluid–rock interaction model may possibly extend to other orogenic gold deposits in the West African Craton.

Peralkaline granites and pegmatites are a prime repository of REE and HFSE, critical raw materials. Although it is accepted that magmatic processes are fundamental in concentrating these metals, the role of hydrothermal fluids in concentrating and fractionating these elements remains unclear. This paper investigates the global reproducibility of the magmatic-hydrothermal evolution of alkaline silica-saturated systems using alkali pyroxene and amphiboles from six alkaline complexes. These minerals contain significant amounts of REE and other HFSE, and pyroxene is stable throughout the magmatic and hydrothermal stages. Amphibole consists of mostly unzoned arfvedsonite, leakeite, and katophorite, while pyroxene is always aegirine. Two types of aegirine were defined. In all complexes, type-I aegirine is zoned; its core is enriched in Ca, REE, Zr, Hf, Sc and Sn, and the rims in Na, Fe(3+)and contains secondary rare-metal bearing minerals and fluid inclusions. Type-II aegirine replaces amphibole and is oscillatory zoned. We interpret the amphiboles and REE-rich cores of type-I aegirine to have grown during the magmatic stage, whereas the rims of REE-poorer type-I and II aegirine are formed during the hydrothermal stage. During magmatic crystallization, REE intake into amphiboles and pyroxene as well as LREE-HREE fractionation were favored by their crystallographic properties and by competition among them and other minerals. During subsequent hydrothermal stages, REE and other HFSE were remobilized, locally reconcentrated and fractionated in mineral pseudomorphs and secondary pyroxene. These observations point out the importance of studying rock-forming minerals such as pyroxenes and amphiboles to unravel geological events controlled by common processes globally.